A printed grayscale extends at one end from no ink (which shows as white on a white print media) through various shades of gray to full black at the other end of the grayscale.
It is often desirable or necessary to apply a “gray neutrality calibration” correction to a printer device's color space in order to have the printer produce visually neutral grays with some well defined digital inputs, for example equal values of cyan, magenta, and yellow. That is, the values C=M=Y. A common way to achieve this is to apply a 1-dimensional “transfer function” to each of the cyan, magenta and yellow colorants separately. Such transfer functions can be calculated based on the colorimetric or densitometric measurements of a printer's “raw” behavior.
However, an undesirable side effect of using 1-dimensional transfer functions is that they not only affect the gray axis (C=M=Y) but also, the rest of the printer's gamut, i.e. the range of colors, that can be obtained by mixing three or more inks. This can lead to non-linearities, contouring, difficulties in creating color maps, gamut reductions, and other artifacts.
There is a problem in printing grayscale images on an inkjet printer device, using the conventional gray neutrality calibration, that the images do not appear to be gray to the naked eye, and are tinted with a non neutral color, that is the gray looks reddish, brownish, yellowish or greenish. In practice, ensuring a neutral gray color is printed is a non trivial problem, especially when working with a 4 color printer, or a 6 color printer, such as the Hewlett Packard DesignJet 5000 type printer.
Ideally, the gray neutrality calibration should be applied only to the gray axis, but without creating discontinuities elsewhere.